The objective of the proposed research is to study the influence of nonexchanging ligands such as macrocyclic ligands on the aqueous proton NMR relaxation enhancement by paramagnetic metal ions. The metal ions Mn(II), Cu(II) and Gd(III) are chosen for this study because they have optimal properties (long electron-spin relaxation time and rapid aqueous exchange rate) for aqueous proton NMR relaxation and also are the metals most likely to be applied for medical diagnostic NMR imaging contrast agents. Macrocyclic ligands are chosen because they form highly stable complexes which are kinetically inert to exchange in solution and provide one or more coordination sites remaining for aqueous ligand exchange. In addition, macrocyclic ligands are synthetically flexible and can be utilized to systematically vary coordination geometry, coordination number and degree of unsaturation. All of these features are important in determining the relationship between relaxation enhancement ability (relaxivity) of a given metal ion per exchanging water coordination site and the nature of other nonexchanging ligands. The influence of nonexchanging ligands has not previously been studied. Preliminary evidence outlined in our laboratory shows that for Mn(II), unsaturation in the macrocyclic ligand can cause variations in relaxivity per water molecule over a range of a factor of 2. These results will be extended to other metal ions and ligands and will provide valuable insights into the mechanism of aqueous proton NMR relaxation by paramagnetic complexes. The results will have applications for the design of improved medical NMR imaging contrast agents and for the interpretation of aqueous proton NMR relaxation enhancement by metalloproteins in relation to their active-site ligands and structure.